Free Thyroxine (FT4) Test

Free thyroxine (FT4) testing measures the level of the free (unbound) and biologically active form of thyroid hormone T4 (thyroxine) in blood serum. T4 is the main hormone secreted by the thyroid gland, serving as a long-term reservoir and primary product, a portion of which is converted in peripheral tissues to the more potent hormone T3. FT4 measures the amount of hormone available for cells to use or convert, making it a direct and reliable indicator of thyroid function.